METAL-HYDRIDES - A REVIEW OF GROUP-V TRANSITION-METALS - NIOBIUM, VANADIUM AND TANTALUM

The group V transition metal (Nb, V, Ta)-hydrogen systems are reviewed . Consideration is given to the different types of hydrides formed, th eir crystallography, thermodynamic data and hysteresis effects on hydr iding/dehydriding. The group V transition metals do not themselves rea ct with hydrogen at room temperature, but quite readily react with hyd rogen when alloyed with other bcc metals of smaller atomic radii. Ther e are uncertainties regarding the structures of hydrides formed by thi s group of metals. These uncertainties mainly arise because of the lim ited number of neutron diffraction studies of the hydrides. Monohydrid es have been reported for Nb, V and Ta, while dihydrides have been rep orted only for Nb and V. Thermodynamic data show that the niobium-hydr ogen solid solution is more stable than the vanadium- or tantalum-hydr ogen solid solutions. Hysteresis have been seen in metal-H systems in this group and, as yet there is no complete explanation for its occurr ence.